The present invention relates to the field of depolymerization and purification of contaminated polyethylene terephthalate. The present invention also relates to the fields of dimethyl terephthalate production, polyethylene terephthalate production, acetic anhydride production, and cellulose ester production.
It is increasingly important to provide economically feasible processes for recycling waste. One such waste is contaminated, but otherwise recyclable, polyethylene terephthalate (PET). This includes PET that is contaminated with a material present on the polymer surface or diffused into the polymer, PET that is copolymerized with modifying agents, and PET that is formed into an article containing layers or coatings of other materials. Much recyclable PET is contaminated to the extent that it must be depolymerized and purified prior to being recycled for use as food packaging. Common contaminants include colorants and dyes, opacifiers, polymerization catalyst metals, polymer modifiers, barrier resins, and oxygen absorbers. The predominant source of recyclable PET is discarded PET soft drink bottles. Scrap PET fiber, scrap PET film, and poor quality PET polymer are also major sources of recyclable PET.
PET has been depolymerized through various chemical reactions including hydrolysis, methanolysis, ammoniolysis, and acetolysis. Acetolysis can be accomplished with either acetic acid itself or with metal salts of acetic acid. For example, JP 53063338 teaches heating PET with acetic acid and an ester-interchange catalyst to form terephthalic acid (TPA) and ethylene glycol diacetate. DE 19534276 discloses a process of digesting scrap PET in a metal acetate melt to form a metal terephthalate ester and ethylene glycol diacetate. Additionally, DE 19534276 teaches recovering the solid terephthalate salt and reacting it with acetic acid to form terephthalic acid and a metal acetate. The metal acetate is recovered for reuse in the metal acetate melt step. However, DE 19534276 further teaches that the ethylene glycol diacetate by-product is not useful in the PET recycling process and should be collected for use as a formulation material in pharmaceutical or cosmetics manufacture, thus producing a wasteful by product.
Another problem commonly encountered in PET recycling is that the conditions required to manufacture PET often produce deleteriously high concentrations of diethylene glycol (DEG) in the polymer chain. DEG copolymerization deleteriously lowers the melting point and strength of PET polymer, that is, both the polymer melting point and strength decrease with increasing DEG copolymerization.
Yet another problem prevalent in the PET recycling field is the lack of sufficient economic incentive for companies to recycle PET or use recycled PET. It would be desirable to provide a PET recycling process that could be easily and efficiently combined with another manufacturing process by utilizing byproducts produced when recycling PET. Such a combination would reduce the cost of recycling PET, increase the amount of PET recycled, and thus produce a positive impact on the environment by reducing the amount of otherwise recyclable PET that is incinerated or landfilled.
In light of the above, it would be desirable to provide a process for depolymerizing and purifying recyclable PET that would produce a recycled PET product with a low concentration of copolymerized DEG. It would be further desirable for such a process to be combined with other chemical processes in such a manner that substantially no waste process byproducts are produced.
The present invention includes a process for depolymerizing and purifying recyclable PET including a step of conducting acetolysis on recyclable PET to produce TPA and ethylene glycol diacetate, a step of converting the TPA to dimethyl terephthalate (DMT), and a step of contacting the ethylene glycol diacetate and DMT under transesterification and polycondensation conditions to form a PET product. The present invention furthermore includes a combination process that further comprises a step of carbonylating the methyl acetate co-product from the PET production step to form acetic anhydride, a still further step of reacting the acetic anhydride with cellulose to form cellulose acetate and an acetic acid by-product, and an even still further step of routing this acetic acid by-product of cellulose acetate production back to the PET acetolysis depolymerization step as a reactant.